Light emitting diodes (LEDs) are commonly made on a semiconductor wafer using a chemical vapor deposition process such as MOCVD. The wafer temperatures in MOCVD fabrication are fairly high, around 800° C.-1000° C., which can lead to problems of uniformity across the wafer. The problems arise because the temperature may not be uniform across the wafer and, in the case of some specific material combinations, the process temperature may be sufficiently high to re-evaporate one or more of the constituent materials. For example, in the case of nitride based LEDs that use indium, such as GaInN LEDs that emit in the blue/green, the process temperature is higher than the re-evaporation temperature of the indium, with the result that the fraction of indium is not uniform across the wafer. This nonuniformity in the indium fraction results in a nonuniformity in the output wavelength of LED devices across the wafer.
This variation in the output wavelength introduces a significant cost to LED manufacturing, since the resulting LED devices produced from the wafer have to be tested and binned according to wavelength. Also, LED users either have to design systems that are tolerant of the variation in peak wavelength or pay the premium costs of binned LEDs.
There is, therefore, a need to reduce the wavelength nonuniformities of LED devices across a wafer.